This invention relates to aligning and centering means for the arms of fork-shaped plate-carriers during the loading and unloading of plates into and from the carriers.
In the production of printed plates and in particular of printed electric and electronic circuit boards, there is a rapidly growing demand for far-reaching automation of all manufacturing steps, in order to reduce production costs and remain competitive. It has, therefore, become necessary to make not only certain production steps fully automatic, but also the conveyance of goods between the various work stations is increasingly required to be fully automated.
Among the various known conveying systems, those using fork-shaped plate carriers of basically U-shaped configuration have found general acceptance. These carriers are mounted fixedly on a suitable, mostly upwardly and/or downwardly moving mechanism driven to move intermittently, with stationary intervals in between.
A forked plate carrier has its one end constituted by a cross bar attached to the moving mechanism, e.g. the chain links, being horizontally aligned, i.e., in a plane vertical to the conveying direction, of two conveyor chains and the other open end constituted by two parallel fork arms in essentially the same plane. The moving mechanism may comprise a single one or a plurality of such forked plate carriers.
In U.S. Pat. No. 1,736,866 of Charles Wagner et al, issued Nov. 26, 1929 there is described a drying apparatus for wet plates which comprises an elevating stretch, in the form of two conveyor chains running about top and bottom sprockets. Forked plate carriers of the above-described type are attached with their closed carrier ends to the conveyor chains.
Charging means are mounted on a framework at a determined level near the lower end of the ascending stretch next to the free fork ends of the passing plate carriers and are adapted for shifting a plate to be dried edgewise into supporting contact with a plate carrier which has stopped during its intermittent upward move at approximately the same level as the plate in the charging means.
This known apparatus comprises transfer means adjacent the top of the ascending stretch of the conveying mechanism, which transfer the plate horizontally from the ascending to an aligned plate carrier at the top of the descending stretch.
Plate removing means similar to the above-mentioned charging means are provided at the foot end of the descending stretch of the conveyor mechanism.
Similar apparatus are described in U.S. Pat. Nos. 3,378,131 and 4,058,908 of Erich Weber, granted Apr. 16, 1968 and Nov. 22, 1977, respectively.
In all of these known apparatuses, a plate being dried is shifted at the top of the straight stretches of the conveyor mechanism from an ascending to a descending stretch, whereby the plates pass through the descending stretch with the same plate face uppermost as in the ascending stretch. This is acceptable when only one face of the plate is partially coated or printed on. However, in particular in the manufacture of electronic circuit boards, but also in other fields of application, it is necessary to coat or print partially both faces of a plate or board, made of paper, card sheet, plastic film, timber, sheet metal or other material as may be required in the particular type of application.
In that case, it is desirable to have the plates turn with their respective carriers, e.g. about the sprockets at the top end of the ascending stretch so that, in the descending branch the dried top face of the plate having dried in the ascending stretch becomes the underside in the descending stretch. Having left the first drying station the former underside, now the top side of the plate can then be partially coated or printed on and pass through another apparatus of the above-described kind, whereupon the plate, dried on both faces, can be discharged from the second apparatus.
In the plant for manufacturing the last-described kind of plates, the latter must not be transferred substantially horizontally between the top ends of the straight ascending and descending stretches of the conveying mechanism but must turn once over as do the plate carriers anyway.
The above-described known apparatus would not be suited for doing so as the plates are deposited loosely on the forked plate carriers.
Therefore, plate carriers have been constructed the two fork arms of which have longitudinal grooves in those of their sidewalls, facing toward each other, which grooves open forward out of the free ends of the two fork arms; thereby a plate to be secured in the carrier can be inserted from the free ends of the two fork arms, which ends are slightly flared laterally outwardly in order to facilitate the introduction of the leading edge into the open ends of the two opposing grooves. The rims of the plate extending parallel with the two fork arms are thus held firmly in the grooves on the inner sidewalls of the two fork arms.
Of course, the two marginal regions of a plate to be introduced into the longitudinal grooves of the fork arms must not be coated or printed on lest they stick in the plate carrier when drying and obstruct removal. Plates thus introduced into the plate carriers will of course be turned over with the carriers. However, it was difficult to fulfill the unavoidable condition that the free ends of the fork arms must be arrested accurately opposite the leading edge of the plate to be inserted into their grooves, and moreover, that the entire fork arms and the entire length of the plate are in perfect or at least near perfect alignment with each other, at least substantially in the same plane, as, otherwise, there is danger that the opposite rims of the plate will bind in the grooves of the fork arms and the only partially, e.g. by half, inserted plate will become stuck in the carrier.
This absolute requirement is difficult to meet in practice, as a different number of plates may be loaded on to a number of carriers in the ascending branch, if an elevated drying chamber is used relative to the plane of plate introduction. Consequently, due to a continuously varying load on the ascending and subsequently on the descending stretch, the conveyor chains will show different amounts of play. Of course, if a sunk drying chamber is used the load will be applied first to the descending and then to the ascending stretch.
Moreover, due to climatic or other influences, the fork arms may become warped in time relative to their closed end bars by means of which they are firmly attached to the conveying mechanism.
For this reason I have seen centering devices provided to engage the two fork arms of a carrier in the arrested position of the latter ready to be loaded. These devices comprised a pin projecting on each of the outside walls, turned away from each other, of the fork arms, in their middle regions between their fixed and their free ends, and, in an aligning device, an engaging centering member having a slot or groove therein with a flared entry opening at one end of the member facing forward or backward, relative to the location in which the fork arms bear their projecting pins . These two members of the aligning device provided firmly stationary in the framework, on the right-hand and left-hand outer sides of the respective two fork arms of the carrier, can be moved parallel with the direction in which the fork arms extend and can thereby catch the single pins on their respective sides firmly. However, I have found that the range within the flared slot opening of the centering member destined to catch a deviating pin on a badly aligned fork arm was not wide enough to catch the fork arm pin on one side, or the two fork arm pins on the outsides of the two fork arms. For it is not possible to enlarge the size of the pin-catching flared opening of a centering member at random as, otherwise, a pin of another fork arm, of a carrier above or below the one to be loaded, might be caught, as the distances between plate carriers in stacked arrangement in the ascending or descending stretches should be kept quite small, to increase the output rate of the apparatus.
Therefore, it would happen repeatedly that the earlier apparatus, the operation of which I had the occasion to supervise, suffered from the drawback that the centering member would hit with the frontal, vertical portion of its forward face, against the pin on the fork arm to be engaged, without the pin entering the outwardly flared recess which would have led it into correct position in the groove of the member. On the contrary, abutment of the vertical forward face of the centering member against the fork arm pin would cause bending of the fork arm and thus even greater misalignment and damage to the conveyor mechanism.
When the misalignment between the two fork arms of a carrier and the plate to be inserted thereinto was even greater, the pins on the two fork arms would miss the centering members completely and the advancing plate to be dried would come to lie on top or below of the carrier into whose fork arm grooves it should have been inserted.
The entire plate-conveying process would thus be seriously disturbed and interrupted. Plate carriers whose two fork arms have been distorted can cause jamming of the plate to be dried or cause excessive resistance to the complete introduction or withdrawal of the plate from its carrier. When the inner rearward regions of at least one of the fork arms of a carrier have become strongly deformed, it may happen that, even when the engagement of the centering pins in the centering members has been successful, the plate moving toward the carrier will miss the entrances to the fork arm grooves entirely.
A consequence of these effects is unavoidably an interruption of the operation of the entire apparatus, requiring exchange of strongly deformed carriers, and leading to damaged plates which have to be discarded as production waste.
As another problem I observed that the fact that plate carriers were exposed during operations to large fluctuations of temperature, e.g. in the case of conveyance of the plates through freezing, heating and burning stations, causes considerable changes in the dimensioning of the soldering or welding spots in the structure of the carrier leading in turn to deformation of the entire fork structure of the carrier.